The most common exposure to mercury—which is both naturally occurring and man-made—is from eating contaminated fish. Other sources: compact fluorescent light bulbs (CFLs), silver dental fillings and batteries. In high doses, mercury can harm the nervous system, heart, lungs, kidneys, and immune system; even in low levels it can affect brains of young children, which is why the U.S. Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA) have issued seafood guidelines for children and pregnant/nursing women.